1. Field of the Invention
This invention relates to a composition and process for coating metallic surfaces; more particularly, it relates to an aqueous composition which may be applied to a metallic substrate in a controlled manner, without subsequent rinsing, prior to the application of an organic siccative finish.
2. Prior Art
In order to obtain the required adhesion of decorative or corrosion-resistive organic films to ferrous or non-ferrous metallic substrates, the use of a chemical pretreatment coating between the metallic surface and the organic film is well known. As all coatings are permeable to moisture to some extent, corrosion of the metallic surface may take place underneath the organic coating which may result in loss of adhesion and the formation of blisters. Should the organic coating become damaged and broken, then underfilm corrosion of the metal surface may take place from this point. An effective chemical pretreatment system may greatly enhance the resistance of the organically-coated metallic substrate to such corrosion.
One particular industry that demands the highest standard of organic coating adhesion and corrosion resistance is the production of pre-painted metal in coil or sheet form, which is subsequently subjected to bending or forming without damage to the coating. The main chemical pretreatment coatings traditionally available to the coil coater are iron phosphates, zinc phosphates, alkaline oxide systems and chromate. In these conventional processes, the chemical reactants are normally applied to the metallic surfaces by immersion or spray in 8-16 seconds at 45.degree.-60.degree. C. As coil coating lines may run in excess of 100 meters per minute, to obtain the required reaction time, the reaction cell or spray section needs to be many meters long and also large holding tanks are needed to provide sufficient capacity so that the reaction products of the pretreatment process do not form an excessive proportion of the bath thereby necessitating frequent dumping and correspondingly causing large fluctuations in the constituent concentrations of the bath and associated difficulties in controlling the process.
These processes vary with regard to efficiency and produce varying quantities of by-products in the form of sludge or scale which may adhere to pipework or block spray jets. Bath control often has to be maintained within close limits and may be relatively involved. After the application of the coating, excess reaction products need to be removed by rinsing, and the rinse water then has to be treated so that toxic or undesirable materials are removed prior to disposing to effluent. It is also necessary to discard all or part of the coating bath itself when reaction products reach a certain level. Again, this requires that toxic or undesirable matter has to be removed prior to disposal to effluent. These toxic materials themselves have to be disposed of and cause the inevitable pollution of the environment.
To avoid a number of the above operational problems and particularly the creation of undesirable effluent, so-called `no-rinse` systems have been introduced. These systems are so-called because, after the application of the appropriate chemical reactant to the metallic surface, the excess is removed by an appropriate means and returned to a reservoir to be reapplied to the metal surface. No subsequent rinsing is required and therefore there is the environmental benefit of no polluting effluent. There are two main types of `no-rinse` systems presently available.
The first type is a reactive system which has been termed `reacted-in-place`. This system comprises highly active chemical solutions that react with the metallic substrate in a very short time to produce the desired coating, see, for example, U.S. Pat. No. 4,266,988. By ensuring a highly active chemical solution, the time required to achieve the desired weight of coating is kept low and allows a short pretreatment section in the production line. Reactive systems inevitably produce a build-up of reaction products in the coating solution, therefore consistency and control of the solution is difficult. Furthermore, sludging of insoluble salts may occur in the circulation system if line speeds vary excessively blocking spray equipment or contaminating the substrate surface. Clearly, as there is no post-rinse, any reaction products or contaminants will remain on the metal surface and may detract from adhesion or corrosion resistance characteristics of a subsequent organic coating.
The second type of no-rinse chemical pretreatment system is the non-reactive system as disclosed in U.S. Pat. No. 4,183,772 and U.S. Pat. No. 4,227,946, for example, whereby the pretreatment solution does not react chemically with the metallic surface. The coatings formed in these processes rely primarily on adsorption on the metallic substrate as the adhesion mechanism. To achieve this, the solutions contain organic or inorganic film-forming agents and wetting agents and the solutions must be applied uniformly over the entire surface at a given coating density. This usually necessitates the use of a roll-coater for film application, whereby the wet film thickness may be controlled. After application, the film requires drying, normally at 100.degree.-250.degree. C., which, due to the relatively high film thickness of such coatings, requires the use of a purpose-built oven or hot air driers.
In both reactive and non-reactive no-rinse systems, the effectiveness of the pretreatment solution may be governed by the extraneous ingredients which do not contribute to the siccative organic coating adhesion and corrosion resistance. In this respect, the addition of film-forming and wetting agents to a non-rinse pretreatment solution is undesirable since they may adversely effect the long term adhesion properties of the subsequently-applied siccative coating, especially under conditions of high humidity. Similarly, the reaction products which build-up in the solution of the reactive-type no-rinse systems may be considered as extraneous ingredients which may reduce the efficacy of the pre-treatment system.